Digital systems frequently require communication with remotely located input-output (I/O) devices. The I/O devices must communicate with a master device as well as between themselves over electrical cables or other mediums. For example, a data processing system generally comprises a master controller with one or more devices connected to it. Typical devices include, but are not limited to, magnetic disk drives, magnetic bubble memories and solid state storage devices. The sophistication and speed of these devices are limited by conventional communication links.
A prior art technique to communicate information between devices includes converting parallel information into a serial stream of electrical pulses and transmitting the stream sequentially through an electrically conductive medium such as a twisted-pair or coaxial cable. On a receiving end, the serial stream of pulses are reconstituted back into parallel form suitable for data processing use. This technique works relatively well with slow data rates but becomes less than ideal at data rates now required. One problem with utilizing mediums such as a twisted-pair or coaxial cable is that the bandwidth of the communication channel is limited due to the inductance, capacitance and resistance per unit length. Another problem is the so-called "ground loop" problem. That is, the "grounds" of the various interconnected devices are not precisely at the same potential. The differences in potential result in currents in the interconnecting cable grounds, adversely affecting communication. Further, such ground currents cause electromagnetic radiation which further impede accurate communication.
Optical digital communication systems have been developed which obviate ground loops and permit large amounts of information to be transmitted over a single channel. While optical communication systems have a tremendous advantage in comparison to electrical or electromagnetic mediums, they are nevertheless subject to problems such as noise and signal attenuation. Means for overcoming these impairments include encoding the information to be transmitted in a manner which allows detection of information which has been corrupted. These encoding methods can be complex and require substantial overhead thus slowing down the effective information transfer rate.
In the present invention, a digital encoding method and apparatus suitable for high speed serial data transfer requiring minimal overhead is disclosed.